How can they go from nice puppy to eat your face off with no prior signs of aggression?

Honestly - they don't. There are ALWAYS signs. You just have to know how to read them.

For example - Fifi is a great dog. She has never been to an obedience class but she goes out with us to the petshop and we have people to the house. If Fifi is laying on the bed, she will growl at us if we try to move her. So we just get in around her. Fifi doesn't like it when people get too close when she is eating, so we don't bother her. Fifi always lets us know when she wants attention, so we give it to her. Fifi doesn't like the vet. It is very cute how she nips at him. She is very little though, and those shots hurt. And I don't like it when people poke and prod at me, so I can't blame Fifi that she doesn't like being poked and prodded at.

Fifi is a dog bite waiting to happen. She runs her family and chances are that if she is pushed to do something she doesn't want to do, she will use her teeth to reinforce HER rules.